In the field of wireless power and data transfer, inductive coupling has been used to provide power to and communicate with a device without making electrical contact. This technique has been used, for example, with implanted medical systems. For implantable systems, multiple medical devices can be implanted inside of the body of a patient. Medical systems utilizing this technique have an external control unit, such as a telemetry controller (TC), and one or more medical devices implanted within the body of a patient. Power transfer and data communication between the external control unit and implanted medical device(s) are provided via an inductive link.
For example, as illustrated in FIG. 1, a conventional power/data transfer system 10 typically includes an external TC 12 capable of performing a medical function (which could be diagnostic and/or therapeutic) and a plurality of implantable medical devices (“implants”) 14, (only two implants 14(y), 14(z) are shown for purposes of brevity in illustration), each of which is capable of sensing physiological signals in the body of a patient and transmitting representative data to the TC 12 in furtherance of performing the medical function.
A primary coil Lp located inside the TC 12 inductively couples and powers secondary coils Ls(y), Ls(z) respectively inside the implanted medical devices 14(y), 14(z). Power is delivered to the implanted medical devices 14 by applying an alternating current (AC) current on the primary coil Lp at a selected transmission frequency Ft. Capacitors Cs(y), Cs(z) are respectively coupled in parallel to the secondary coils Ls(y), Ls(z) to form LC tank circuits that are tuned to resonant at the transmission frequency Ft. In addition to providing power to the medical devices, the coils Lp's and Ls's are also utilized for communication between the TC 12 and the implanted medical devices 14. For downlink data from the TC 12 to the implanted medical devices 14, different modulation techniques can be applied to the AC current on the primary coil Lp.
For uplink data from the implanted medical devices 14 to the TC 12, a load modulation technique can be used. In this technique, each implanted medical device 14 transmits uplink data to the TC 12 in a given time slot in a time-division multiplexed manner by modulating a load resistance Rs to a modified load resistance Rs+ΔRs according to the uplink data, where ΔRL is the amount of change on the load resistance. Due to the inductive coupling between the primary coil Lp and the corresponding secondary coil Ls, a voltage amplitude change on the primary coil Lp according to the uplink data is obtained. Based on the amplitude change, the TC 12 can demodulate the data sent from a particular implanted medical device 14 at the corresponding time slot utilizing any one or more of a variety of demodulation techniques, including amplitude shift keying (ASK), phase shift keying (PSK), frequency shift keying (FSK), etc.
The amplitudes of the signals received by the TC 12 from the implanted medical devices 14 may different from each other. For example, depending on the distances, as well as the characteristic of the material, between the primary coil Lp and the secondary coils Ls(y), Ls(z), the coupling coefficients Kc(y), Kc(z) between the primary coil Lp and the respective secondary coils Ls(y), Ls(z) can be different for the different implanted medical devices 14(y), 14(z). The difference in the respective coupling coefficients Kc(y), Kc(z) between the primary coil Lp and the secondary coils Ls(y), Ls(z) will affect the voltage amplitudes on different secondary coils Ls(y), Ls(z). Furthermore, if each medical device 14 utilizes the same amount of load resistance change ΔRs for load modulating the uplink data, the voltage amplitude induced on the primary coil Lp for each implanted medical device 14 will also be different. These voltage amplitude differences on the primary coil Lp due to different coupling coefficients Kc(y), Kc(z) will complicate the circuitry inside the TC 12 that demodulates the uplink data from the induced voltage on the primary coil Lp. Thus, the received signal amplitudes corresponding to the respective implanted medical devices 14(y), 14(z) may be primarily affected by the coupling coefficients Kc(y), Kc(z). The received signal amplitudes corresponding to the respective implanted medical devices 14(y), 14(z) may also be secondarily affected by the different tuning tolerances between the primary coil Lp and the respective secondary coils Ls(y), Ls(z).
For example, referring to FIG. 2, the changes in the amplitude of AC voltage induced on the primary coil Lp due to load modulations at the secondary coils Ls are represented as changes in an envelope signal Senv. A simple demodulator design utilizes an envelope detector to extract the envelope signal Senv from the amplitude changes induced on the primary coil Lp, and a comparator to compare the envelope signal Senv with an appropriate threshold level Sth to determine the uplink data. In the embodiment illustrated in FIG. 2, an ASK modulation technique is employed to encode the envelope signal Senv with data that can then be demodulated to acquire the data therefrom.
For example, as shown in FIG. 3a, an ASK modulated envelope signal Senv1, which contains one of two bits of information (“1” or “0”) during each symbol period (indicated between the dashed lines), can be converted into a digital signal by comparing the envelope signal Senv1 to a threshold level Sth. The data value can be read as switching between “0” and “1” if and when the envelope signal Senv1 crosses the threshold level Sth in the respective symbol period, i.e., from “0” to “1” when the envelope signal Senv1 rises above the threshold level Sth, and from “1” to “0” when the envelope signal Senv1 falls below the threshold level Sth.
In an alternative embodiment shown in FIG. 3b, a four-phase (0°, 90°, 180°, and) 270° PSK modulated envelope signal Senv2, which contains two bits of information (“00,” “01,” “10,” and “11”) during each symbol period (indicated between the dashed lines), can be converted into a digital signal by comparing the envelope signal Senv2 to a threshold level Sth. The data value can be read as being “00,” “01,” “10,” and “11,” depending on when and in what direction the envelope signal Senv2 crosses the threshold level Sth in the respective symbol period.
In still another alternative embodiment shown in FIG. 3c, an FSK modulated envelope signal Senv3, which contains one of two bits of information (“1” or “0”) during each symbol period (indicated between the dashed lines), can be converted into a digital signal by comparing the envelope signal Senv3 to a threshold level Sth. The data value can be read as “0” and “1,” depending on how many times the envelope signal Senv3 crosses the threshold level Sth in the respective symbol period, i.e., a “0” if the envelope signal Senv crosses the threshold level Sth three or less times (resulting from the relatively low-frequency portion of the envelope signal Senv), a “1” if the envelope signal Senv cross the threshold level Sth more than three times (resulting from the relatively high-frequency portion of the envelope signal Senv3)
Regardless of the type of demodulation technique, when the coupling coefficients Kc between the primary coil Lp and the secondary coils Ls(y), Ls(z) of the implanted medical devices 14(y), 14(z) differ, the peak-to-peak amplitudes of the envelope signals Senv on the primary coil Lp for the implanted medical devices 14(y), 14(z) will be different. In this case, the peak-to-peak amplitude of the envelope signal Senv for the implanted medical device 14(y) with a relatively high coupling coefficient Kc(y) will be greater than the peak-to-peak amplitude of the envelope signal Senv for the implanted medical device 14(z) with a relatively low coupling coefficient Kc(z). Thus, different threshold level values St(y), St(z) are respectively required to correctly demodulate the uplink data for the implanted medical devices 14(y), 14(z).
Because a single threshold level value St cannot be used to demodulate the uplink data from the different implanted medical devices 14, a more complicated demodulator design utilizing equalization techniques for the envelope signals Senv is required. If the coupling coefficients Kc drift in time, an even more complicated demodulator design using adaptive equalization will become necessary. Alternatively, AC coupling can be used between the envelope detector and the comparator, such that the average value of the envelope signal Senv for the uplink data sent by the different implanted medical devices 14 will move to ground, and thus, the threshold level St can be set to ground. The uplink data can therefore be correctly demodulated from the envelope signal Senv. However, because it will take some time to have the average value of the envelope signal Senv to move to ground at the output of the AC coupling whenever a different implanted medical device sends out uplink data, the data within the time required for settling the average value of the envelope signal Senv to ground cannot be reliably detected without significantly reducing the uplink data transmission rate.
There, thus, remains a need for providing a simpler means that allows demodulation of uplink data sent from multiple implantable medical devices without having to reduce the uplink data transmission rate.